Lead zirconate titanate is a typical piezoelectric material and is used in various piezoelectric devices, such as actuators, oscillators, sensors, and filters. However, it has been pointed out that when lead-containing piezoelectric devices are scrapped and exposed to acid rain, lead components may leach from the piezoelectric materials into the ground and harm an ecosystem. Thus, lead-free piezoelectric materials for lead-free piezoelectric devices are being actively studied and developed.
NPL 1 discloses that, in a solid solution of a small amount of barium titanate in an antiferroelectric substance sodium niobate, sodium niobate converts into a ferroelectric substance. The sodium niobate material described in NPL 1 is free of lead and potassium. Potassium is responsible for poor sinterability and low moisture resistance. The material described in NPL 1 has a barium titanate concentration in the range of 5% to 20% and is sintered at a temperature in the range of 1200° C. to 1280° C. The Curie temperature of the sodium niobate material described in NPL 1 is higher than the Curie temperature (120° C. to 130° C.) of a typical lead-free piezoelectric material barium titanate. For example, it is disclosed that (Na0.9Ba0.1)(Nb0.9Ti0.1)O3 has a Curie temperature of 230° C. However, this composition has a mechanical quality factor as low as 140.